487 research outputs found
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
MgII absorption systems with W_0 > 0.1 \AA for a radio selected sample of 77 QSOs and their associated magnetic fields at high redshifts
We present a catalogue of MgII absorption systems obtained from high
resolution UVES/VLT data of 77 QSOs in the redshift range 0.6 < z < 2.0, and
down to an equivalent width W_0 > 0.1 \AA. The statistical properties of our
sample are found to be in agreement with those from previous work in the
literature. However, we point out that the previously observed increase with
redshift of dN/dz for weak absorbers, pertains exclusively to very weak
absorbers with W_0 < 0.1 \AA. Instead, dN/dz for absorbers with W_0 in the
range 0.1-0.3 \AA actually decreases with redshift, similarly to the case of
strong absorbers. We then use this catalogue to extend our earlier analysis of
the links between the Faraday Rotation Measure of the quasars and the presence
of intervening MgII absorbing systems in their spectra. In contrast to the case
with strong MgII absorption systems W_0 > 0.3 \AA, the weaker systems do not
contribute significantly to the observed Rotation Measure of the background
quasars. This is possibly due to the higher impact parameters of the weak
systems compared to strong ones, suggesting that the high column density
magnetized material that is responsible for the Faraday Rotation is located
within about 50 kpc of the galaxies. Finally, we show that this result also
rules out the possibility that some unexpected secondary correlation between
the quasar redshift and its intrinsic Rotation Measure is responsible for the
association of high Rotation Measure and strong intervening MgII absorption
that we have presented elsewhere, since this would have produced an equal
effect for the weak absorption line systems, which exhibit a very similar
distribution of quasar redshifts.Comment: Accepted for publication in ApJ. 12 pages, 8 figure
Resolving the Stellar Outskirts of M31 and M33
Many clues about the galaxy assembly process lurk in the faint outer regions
of galaxies. The low surface brightnesses of these parts pose a significant
challenge for studies of diffuse light, and few robust constraints on galaxy
formation models have been derived to date from this technique. Our group has
pioneered the use of extremely wide-area star counts to quantitatively address
the large-scale structure and stellar content of external galaxies at very
faint light levels. We highlight here some results from our imaging and
spectroscopic surveys of M31 and M33.Comment: 6 pages, 4 figures. To appear in the proceedings of "Island Universes
- Structure and Evolution of Disk Galaxies", editor R.S. de Jong (Springer:
Dordrecht
Testing adiabatic contraction of dark matter in fossil group candidates
We present deep XMM observations and ESO WFI optical imaging of two
X-ray-selected fossil group candidates, RXCJ0216.7-4749 and RXCJ2315.7-0222.
Using the X-ray data, we derive total mass profiles under the hydrostatic
equilibrium assumption. The central regions of RXCJ0216.7-4749 are found to be
dominated by an X-ray bright AGN, and although we derive a mass profile,
uncertainties are large and the constraints are significantly weakened due to
the presence of the central source. The total mass profile of RXCJ2315.7-0222
is of high quality, being measured in fifteen bins from [0.075 - 0.75]R500 and
containing three data points interior to 30 kpc, allowing comprehensive
investigation of its properties. We probe several mass models based on the
standard NFW profile or on the Sersic-like model recently suggested by
high-resolution N-body simulations. We find that the addition of a stellar
component due to the presence of the central galaxy is necessary for a good
analytical model fit. In all mass profile models fitted, the mass concentration
is not especially high compared to non-fossil systems. In addition, the
modification of the dark matter halo by adiabatic contraction slightly improves
the fit. However, our result depends critically on the choice of IMF used to
convert galaxy luminosity to mass, which leads to a degeneracy between the
central slope of the dark matter profile and the normalisation of the stellar
component. While we argue on the basis of the range of M_*/L_R ratios that
lower M_*/L_R ratios are preferred on physical grounds and that adiabatic
contraction has thus operated in this system, better theoretical and
observational convergence on this problem is needed to make further progess.Comment: to appear in Astronomy & Astrophysics, 16 pages, 24 figure
Optical properties and spatial distribution of MgII absorbers from SDSS image stacking
We present a statistical analysis of the photometric properties and spatial
distribution of more than 2,800 MgII absorbers with 0.37<z<1 and rest
equivalent width W_0(\lambda2796)>0.8\AA detected in SDSS quasar spectra. Using
an improved image stacking technique, we measure the cross-correlation between
MgII gas and light (in the g, r, i and z-bands) from 10 to 200 kpc and infer
the light-weighted impact parameter distribution of MgII absorbers. Such a
quantity is well described by a power-law with an index that strongly depends
on W_0, ranging from ~-1 for W_0~ 1.5\AA. At redshift
0.37<z<0.55, we find the average luminosity enclosed within 100 kpc around MgII
absorbers to be M_g=-20.65+-0.11 mag, which is ~0.5 L_g*. The global
luminosity-weighted colors are typical of present-day intermediate type
galaxies. However, while the light of weaker absorbers originates mostly from
red passive galaxies, stronger systems display the colors of blue star-forming
galaxies. Based on these observations, we argue that the origin of strong MgII
absorber systems might be better explained by models of metal-enriched gas
outflows from star-forming/bursting galaxies. Our analysis does not show any
redshift dependence for both impact parameter and rest-frame colors up to z=1.
However, we do observe a brightening of the absorbers related light at high
redshift (~50% from z~0.4 to 1). We argue that MgII absorbers are a phenomenon
typical of a given evolutionary phase that more massive galaxies experience
earlier than less massive ones, in a downsizing fashion. (abridged)Comment: ApJ in press, 28 pages, 16 figures, using emulateapj. Only typo
corrections wrt the original submission (v1
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